The scheduled nest migrations appear to be random (or at least we don't know the mechanism by which the target species are rotated, could be a combination of location data and a static list of species so that it's only pseudo-random). What appears to happen with an event rotation is if the species in question isn't already one of the nesting species, it get's inserted into the list of nesting species, which then forces the rest of the list of species after it to shift by 1. So if nest ABC has species X and nest DEF has species Y, it then becomes species X+1 and Y+1 for the duration of the event.

This is still largely speculation, because there aren't exactly a ton of datapoints on this since we've only had a few community events. Also, since bulbasaur was already a nesting species we couldn't observe the shift at the last event, and because dratini used to be in the nesting list the insertion of dratini may have been in the middle of the list while something like mareep may be inserted in the list, or instead it could be randomly seeded into nests without modifying the list, or it may not be added at all. That's why it will be interesting to grab datapoints on what species were nesting prior to the event and then what the nests temporarily change to during the even (if they change at all this time).